Poor Ted set off a missile (7)
I believe the answer is:
torpedo
'a missile' is the definition.
(missile from a submarine)
'poor ted set' is the wordplay.
'set' is an anagram indicator (instruction to set the letters).
'poor'+'ted'='poorted'
'poorted' with letters rearranged gives 'TORPEDO'.
'off' acts as a link.
(Other definitions for torpedo that I've seen before include "Self-propelled submarine weapon" , "fish you want" , "Anti-ship weapon" , "attack" , "Submarine weaponry" .)